In hard disc drives (HDDs), data is written/read while a magnetic head is floated from the surface of a rotating magnetic recording medium at a gap of several tens of nm by a floating mechanism (slider). Bit information on the magnetic recording medium is stored in data tracks arranged concentrically on the recording medium surface. The data writing/reading head is moved and positioned to a target track on the magnetic recording medium surface at a high speed to write and read data.
A positioning signal (servo signal) for detecting the relative position between the head and the data tracks is concentrically written on the surface of the magnetic recording medium, and the magnetic head for writing/reading data detects the relative position thereof at a fixed time interval. The servo signal, which is written in a magnetic recording medium using a dedicated device (servo writer) after the magnetic recording medium is installed in the HDD, is used to prevent the head from deviating from the center of the servo signal (or the center of the locus of the head) detected by the head.
The recording density of such a magnetic recording medium reached 100 Gbit/in2 in the case of magnetic recording media under current development, and the storage capacity trend is to increase by 60% or more per year. In connection with the increase of the storage capacity, however, there is a tendency for the density of the servo signal for detecting the relative position between the head and the data track by the head to likewise increase, which increases the writing time of the servo signal, year by year. The increase of the writing time of the servo signal is one factor that reduces productivity of HDDs and increases the cost.
Against this backdrop, a magnetic transfer technique has been developed to significantly shorten the writing time of the servo information by collectively writing the servo signal in a magnetic recording medium, as compared with the system of writing a servo signal by using a signal writing head of the servo writer as described above. FIGS. 8A–8C, 9A, and 9B illustrate this technique. This technique involves an initial demagnetizing step where a permanent magnet 82 for demagnetization is moved while kept spaced from the surface of a magnetic recording medium 81 at a fixed interval or gap of 1 mm or less (FIG. 8A). A magnetic layer formed on the magnetic recording medium 81 is not magnetized in a uniform direction before this step. It is uniformly magnetized in a uniform direction indicated by an arrow by magnetic field leaking between the gap of the permanent magnet 82 (FIG. 9A). FIG. 8B shows the master disc positioning step in which a master disc 83 for magnetic transfer is positioned on the magnetic recording medium. JP-A-11-175973 describes a technique for enhancing the reliability to this positioning step using a marker for positioning between the center of the master disc and the center of the medium. FIG. 8C shows the transfer pattern writing step, in which the master disc 83 is brought into close contact with the surface of the magnetic recording medium 81, and the permanent magnet 82 for magnetic transfer is moved along a movement path indicated by an arrow to perform the magnetic transfer.
FIGS. 9A and 9B are diagrams showing the mutual positional relationship between the permanent magnet and the magnetic recording medium in the initial demagnetizing step and the transfer pattern writing step for magnetic transfer. FIG. 9A shows the positional relationship in the initial demagnetizing step and FIG. 9B shows the positional relationship in the transfer pattern writing step. As shown in FIG. 9B, in the transfer pattern writing step, the master disc and the magnetic recording medium are disposed so that a soft-magnetic-film-side surface of the master disc having the soft magnetic film 95, including a Co type soft magnetic layer embedded in one surface of an Si substrate 94, is brought into close contact with a magnetic-layer-side surface of the magnetic recording medium having a magnetic layer 92 on a substrate 91. The permanent magnet 93 for magnetic transfer is swept around the Si substrate 94 to transfer the magnetic pattern onto the magnetic recording medium.
The soft magnetic film 95 having the Co type soft magnetic layer embedded in the form of a pattern is interposed between the permanent magnet 93 and the magnetic layer 92 so that the magnetic field formed in the Si substrate 94 by the permanent magnet 93 can magnetize the magnetic layer 92 at the portions corresponding to the positions of the soft magnetic film 95 where no Co type soft magnetic layer exists. In the magnetic layer 92 at the portions corresponding to the positions at which the Co type soft magnetic layer exists, the magnetic field passes through the soft magnetic film 95 to form a magnetic route having a smaller magnetic resistance, which is sufficiently weaker as to be incapable of writing a new signal. As shown in FIG. 9B, the direction of the magnetic field for writing the transfer signal is opposite to the direction of the magnetic field for demagnetization.
FIGS. 10A–10E illustrate a method of embedding a soft magnetic layer in a master disc. The embedding method involves the steps of coating a resist layer (FIG. 10A), patterning and developing the resist layer (FIG. 10B), etching an Si substrate using the patterned resist layer as an etching mask to form grooves (FIG. 10C), sputtering soft magnetic material on the Si substrate to form a Co soft magnetic film (FIG. 10D), and removing the extraneous Co film outside the grooves (FIG. 10E).
Specifically, as shown in FIG. 10A, a photoresist layer 1002 of 1 μm thick is coated on the surface of an Si substrate 1001 of about 500 μm thick using a spin coater. The resist layer 1002 is then patterned using a conventional photolithography technique, as in the case of the normal method of manufacturing an Si semiconductor, as shown in FIG. 10B. Subsequently, as shown in FIG. 10C, the exposed surfaces of the substrate 1001 is dry etched with a conventional reactive plasma etching technique using methane trichloride as reactive gas so that the surfaces of the Si substrate 1001 not covered by the resist layer 1002 is removed to a depth of about 500 nm by etching. Furthermore, as shown in FIG. 10D, a Co type soft magnetic film 1003a, 1003b of 500 nm thick is formed by sputtering over the resist layer 1002. Finally, as shown in FIG. 10E, the Si substrate 1001 is immersed in a solvent to dissolve and remove the resist layer 1002 and the Co soft magnetic film 1003b on the resist layer 1002 (by using ultrasonic wave or the like as occasion demands).
Presently, a magnetic film is formed on both sides of a magnetic recording medium to increase its storage capacity in HDDs, as described for example in JP-A-2002-319128. A servo pattern as described above and a data pattern are provided as a magnetic pattern to be recorded in the magnetic recording medium. Many magnetic discs are loaded in a present HDD device, and a magnetic head is individually equipped at each side of each magnetic disc. When a data area is identified, only the head stack assembly (HAS) associated with the head corresponding to a cylinder in the data area concerned is controlled to move to a predetermined position under the servo control. The other HSAs are not controlled. For example, when a cylinder to be read/written is varied from the front surface to the back surface, if a displacement between the servo patterns on both the surfaces is large, it takes time until the positioning of the HAS on the back surface by the servo control is stabilized, so that the set ring time corresponding to the reading time of data after the cylinder is switched is increased, degrading the access speed of the HDD. Therefore, it is very important for the magnetic patterns on both sides of the magnetic recording medium to be aligned as much as possible so that there is no displacement between the two magnetic patterns.
A conventional target pattern to be magnetically transferred is limited to a servo pattern having a large pattern width because the photo-process technique for forming master discs is based on “μm-rule.” However, in consideration of advancement of utility of magnetic transfer and the photo-process technique and application of “sub-μm-rule,” a data pattern as well as a conventional servo pattern should be considered as a transfer target, and it is required to carry out the magnetic transfer based on a high-density master disc to a magnetic recording medium.
It is very difficult to position and align the magnetic pattern of a master disc for the front side of a magnetic recording medium with a magnetic pattern of the master disc for the back side of the magnetic recording. The present invention has been implemented to solve this problem. There remains a need for a magnetic recording medium that can easily provide magnetic patterns having no displacement between the two sides of the magnetic recording medium. The present invention addresses this need.